The manufacturing of metals can involve several stages of handling molten metal. Metal compositions may be melted in a first vessel and transferred to one or more vessels during a metal manufacturing process. For example, a metal composition may be first heated in a furnace, and transferred to a ladle and then to a caster or tundish where it is poured into moulds or a final product. At each stage of a metal manufacturing process, the temperature of the melt may be held at a target temperature for a period of time. Appreciable temperature deviations from a target temperature may adversely affect the quality of the final metal product. Accordingly, it is desirable to monitor, as accurately as possible, a temperature of the molten metal through the metal manufacturing process.
Several types of temperature measurement techniques have been developed for measuring high temperatures of molten metals. One approach is to use thermoelectric devices, e.g., thermocouples. Measurements with thermocouples may involve fixing the thermocouple at the end of a long lance, and dipping the thermocouple into the molten metal to measure the temperature of the melt. This is conventionally done in an intermittent fashion. Disadvantages with intermittent measurement of temperature is that the manufacturing process may be interrupted for the measurement, and there may be substantial time intervals between the measurements so that close process control may not be possible.
Optical-pyrometry-based techniques have been developed for measuring the temperature of liquid metals. Such techniques can provide faster measurements of temperatures, although the techniques used also provide intermittent measurements. Conventional optical measurements comprise piercing an optical probe, mounted on a lance, through a layer of slag covering the metal. The slag and measurement technique can lead to high wear and a short lifetime of the probe.
The use of thermocouples for temperature measurements of molten metal compositions may have additional disadvantages. For example, thermocouples are not entirely immune to electromagnetic radiation, and typically have slow response times. In addition, they are not adaptable to repeated continuous measurement applications, since they undergo degradation with time at high temperature. Thermocouples cannot be brought into direct contact with the molten metal and require some protective covering material, such as refractory sheathing, to protect the thermocouple from the high-temperature melt and to prevent the thermocouple signals from being short circuited and dissipated into the molten metal. A protective refractory covering should be both thick enough, so that the thermocouple is not exposed to the melt, and thin enough, so that temperature of the melt can be accurately measured by the thermocouple. Accordingly, there are conflicting requirements on the thickness of the thermocouple's protective covering.
The protective material covering a thermocouple can be worn off by the molten metal with repeated use, which can introduce measurement errors over time. In addition, the erosion rate may depend upon the type of metal being processed, the temperature of the molten metal, and the vessel or environment in which the measurement is made. For example, in the case of molten steel, the rate of erosion is low in the tundish while it can be high in a ladle and the furnace. Erosion through the protective material can render the thermocouple inoperable. Therefore, different thicknesses of protective covering may be required for different metals and environments, making manufacture of a thermocouple temperature probe difficult.